1. Technical Field
The invention may include embodiments that may relate to resonant cavity light emitting devices. The invention may include embodiments that may relate to a method of making and/or using resonant cavity light emitting devices, or components thereof.
2. Discussion of Related Art
Light may be extracted from a light emitting diode (LED). The light emitting diode may emit light of only one wavelength, whereas for many applications at least one additional wavelength or white light may be desirable. The use of phosphors, such as in a polymer matrix, may generate light of additional wavelengths, but may have reduced efficiency by, for example, Stokes shifts, reflective or light-scattering losses, and device failures associated with the phosphor packaging. Resonant cavity devices may provide more efficient light extraction, but it may be difficult to fabricate reliable resonant cavity devices in the group III-nitride material system.
Gallium nitride substrate material may exhibit: (i) a close lattice match which, neglecting dopant effects, may be useful for gallium nitride device layers; (ii) reduced strain and dislocation formation in the epitaxial group III-nitride layers as a consequence of the close lattice match; (iii) chemically abrupt interfaces without problematic interdiffusion; (iv) reduction of anti-phase boundaries; and (v) thermal matching that promotes thermal stability during thermal cycling associated with high epitaxial growth temperatures, during high temperature device processing, or end use.
Using a gallium nitride wafer for growth may include: (i) economy of scale (more devices per wafer); (ii) easier handling; (iii) easier automated machine manipulation; and (iv) the ability to fabricate large-area devices. Resonant cavity light emitting diodes may be relatively smaller to a side.
In spite of these well-known advantages, commercial group III-nitride light emitting devices continue to be grown heteroepitaxially on sapphire or silicon carbide substrates due to a lack of high quality large-area gallium nitride substrates. The chemical passivity of nitrogen, a high melting temperature of gallium nitride, and other factors have heretofore made growth of a large volume and high quality gallium nitride boule problematic.
U.S. Pat. Nos. 5,637,531 and 6,273,948 disclose methods for growing gallium nitride crystals at high pressure and high temperature, using liquid gallium and gallium-based alloys as a solvent and a high pressure of nitrogen above the melt to maintain GaN as a thermodynamically-stable phase. The process may be capable of growing electrically-conductive GaN crystals with a dislocation density of about 103-105 cm−2 or, alternatively, semi-insulating GaN crystals with a dislocation density of about 10-104 cm−2, as described by Porowski, “Near defect-free GaN substrates” [MRS Internet J. Nitride Semicond. Research 4S1, G1.3 (1999)]. However, the conductive crystals have a high n-type background doping on the order of 5×109 cm−3, believed to be due to oxygen impurities and nitrogen vacancies. The high n-type background causes substantial crystal opacity, for example optical absorption coefficients of around 200 cm−1 in the visible range, which may be problematic for flip-chip light emitters, and causes the lattice constant to increase by about 0.01 percent to about 0.02 percent, generating strain in epitaxial GaN layers deposited thereupon. The undoped GaN substrates formed by this method have a carrier mobility of about 30 to about 90 cm2/V-s, which may be problematic in high-power devices.
Another technology for growth of pseudo-bulk or bulk GaN may be hydride/halide vapor phase epitaxy, also known as HVPE. In one approach, HCl reacts with liquid Ga to form vapor-phase GaCl, which may be transported to a substrate where it reacts with injected NH3 to form GaN. The deposition may be performed on a non-GaN substrate such as sapphire, silicon, gallium arsenide, or LiGaO2. The dislocation density in HVPE-grown films may be initially quite high, on the order of 1010 cm−2 as may be typical for heteroepitaxy of GaN, but drops to a value of about 107 cm−2 after a thickness of 100 to about 300 micrometers of GaN has been grown. Heteroepitaxial growth of thick HVPE GaN results in strain-induced bowing during cooldown after growth, which remains even after removal of the original substrate.
In view of the difficulty in producing large gallium nitride boules, some efforts have been directed toward developing complex techniques such as epitaxial lateral overgrowth (ELO) for producing individual gallium nitride substrates. In ELO, an epitaxy-inhibiting mask may be deposited over a nucleation substrate such as a sapphire wafer. The mask may be lithographically processed to define openings. Gallium nitride growth nucleates in and fills the openings, and then grows laterally over the masked areas in a lateral overgrowth mode. ELO material has been shown to suppress dislocation densities. Optionally, the nucleation substrate may be removed and the ELO growth process may be repeated on the free-standing gallium nitride wafer. Some reports claim dislocation densities as low as 104 cm−2 obtained by ELO.
However, much higher dislocation densities remain above the openings where ELO growth initiates. Moreover, coalescence of lateral overgrowth from adjacent openings produces tilt boundaries that may manifest in thick layers as arrays of edge dislocations. Repeated application of epitaxial lateral overgrowth may not be expected to suppress the tilt boundaries. Thus, epitaxial lateral overgrowth may not be upwardly scalable in the lateral wafer dimension, and usable growth dimensions may be limited to about the order of the spacings of the nucleation openings. Furthermore, ELO does not produce a three-dimensional single-crystal boule, and the processing involved in producing each ELO gallium nitride wafer may be labor-intensive, making automation of the ELO wafer formation process difficult.
Doping of GaN by rare earth metals may produce luminescence. For example, Lozykowski et al. (U.S. Pat. No. 6,140,669) disclose incorporating rare earth ions into GaN layers by ion implantation, MOCVD, or MBE, and annealing at 1000 degrees Celsius or greater. Birkhahn et al. (U.S. Pat. No. 6,255,669) disclose fabrication of light-emitting diodes using GaN layers doped with a rare earth ion or with chromium. However, these references focus on thin GaN epitaxial layers rather than bulk crystals and do not relate to resonant cavity devices.
Mueller-Mach et al. (WO 01/24285 A1) disclose the fabrication of GaN-based light-emitting diodes on a single crystal phosphor substrate, preferably, rare-earth-doped yttrium aluminum garnet. DenBaars et al. (WO 01/37351 A1) disclose the fabrication of GaN-based light-emitting diode structures, including a vertical laser structure, on a substrate doped with chromium or other transition or rare earth ions. However, the disclosed laser structure employs only a single cavity and has no capability for directional emission of two or more visible wavelengths of light or of white light.